1. Field of the Invention
The invention relates to a process for purifying alkylphenols obtained by alkylation of phenols in the presence of acid catalysts and to the treatment of the effluent thus obtained.
In the text which follows the alkylphenols and butylphenols obtained from the alkylation, and in particular butylation, of phenols are termed the alkylate and butylate.
2. Discussion of Prior Art
Sulphuric acid is customarily used as the catalyst for the alkylation of phenols, especially of cresols, with isobutene (Ind. Chem. Engng. Volume 35 (1943), page 266). Oleum or phenolsulphonic acids (British patent specification No. 701,438) or SO.sub.3 (DOS (German Published Specification) No. 2,215,452) can also be used.
When the alkylation reaction has ended, the acid catalyst contained in the alkylate must be rendered inactive and/or removed. Usually this is effected by means of a treatment with aqueous sodium hydroxide solution or sodium carbonate solution (DT-AS (German Published Specification) No. 1,145,629 and Ind. Chem. Engng., loc cit., page 269).
The treatment with alkali must be carried out very intensively. If this is not the case it is possible, according to our own experience, for substances which have an acid action to form from potential acid-forming agents during storage of the alkylate or at the high temperatures which are required for the distillation, which substances, in the case of tert.-butyl cresols, lead to the tertiary butyl group being split off again, thereby rendering distillation of the alkylate impossible. However, whether or not a total process comprising further steps can be carried out can depend on whether or not the alkylate can be distilled. This is the case, for example, for the fractionation of di-tert.-butyl cresols, which is of great industrial importance for the separation of mixtures of m-cresol and p-cresol (Ind. Engng. Chem., loc cit., page 271 and Franck, Collin, Steinkohlenteer (Coal Tar) Springer Verlag 1968, page 82).
The removal of the acid and of the acid-forming compounds is effected, for example, by circulating the crude butylate mixture together with an excess of a 5 to 7 percent strength by weight aqueous solution of an alkali metal hydroxide several 100 times under a pressure of 5 bars, at about 120.degree. C. and with an average residence time of 40 minutes. After the aqueous alkali solution has separated out, considerable amounts of water are still dissolved or emulsified in the butylate phase and the latter thus also contains considerable amounts (about 1 percent by weight) of the readily water-soluble alkali metal cresol-sulphonates, as can be seen from Table III (loc., page 272). Furthermore, alkali, which is necessary to ensure that the crude butylate mixture is stable on distillation, is dissolved in the butylate. On distillation, that is to say after the aqueous phase has been separated off, alkali metal cresol-sulphonates, in the main, separate out in the butylate and can lead to considerable breakdowns in operation, especially in the case of continuous distillations. In every case these products, which give a tar-like residue, have an adverse effect, both because of their amount and because of their properties, on the working up, by distillation, of the crude alkylate mixture and there is a direct relationship between the sulphur content, and thus the alkali metal sulphonate content, of the alkylate mixture and its decreasing stability to distillation (Ind. Engng. Chem., loc. cit., page 271).
In order further to remove the alkali metal sulphonates, it has already been proposed to rinse several times with a large amount of water, after the wash with alkali (British patent specification No. 701,438; DT-OS (German Published Specification) No. 2,215,452). However, the low residual alkalinity which is necessary to ensure stability on distillation is also eliminated in this way and the amount of effluent is greatly increased.
The washings, which have an alkaline reaction, represent a particular problem in the process according to the state of the art. They contain, in solution, the sulphonic acids, in the form of the alkali metal salts, in addition to phenols and alkylphenols. However, whilst the dissolved phenols and alkylphenols, in particular the cresols and butyl cresols, can be recovered, at least in part, for example by an extraction with benzene (Ind. Engng. Chem. Volume 35 (1943), page 271), this is not possible in the case of the alkali metal sulphonates, in which several percent by weight of the valuable starting material, that is to say cresol, are bound and which represent the main impurity in the effluent. Elimination of the effluent is also difficult since, as is known, the alkali metal sulphonates cannot be precipitated with calcium oxide in the form of a suspension of calcium hydroxide, the salts which remain on evaporation of the wash waters can not be dumped because of their ready solubility in water and combustion of the effluent is also problematical because of the evolution of sulphur dioxide associated therewith.
The purification of the butylate mixture obtained represents a largely unsolved part of the problem, especially in the case of the industrial process for the butylation of cresols, as part of the separation of mixtures of m-cresol and p-cresol.